Variable focus optical system for data reading

ABSTRACT

An optical system and method for data reading in which a light source generates an optical beam directed toward an object to be read and a variable aperture mechanism positioned in the outgoing light path establishes at least two separate focal planes. The variable aperture device may comprise a variable aperture system in which the size of the aperture is selectively varied about a range within the diffractive limit of the light beam. A preferred aperture mechanism is a liquid crystal aperture with one or more aperture regions which are selectively or consecutively activated. The optical system therefore establishes two or more separate waist locations from a single light source resulting in greater depth of field or multiple depths of field.

RELATED APPLICATION DATA

This application is a continuation in part of application Ser. No.07/992,932, filed Dec. 18, 1992, now U.S. Pat. No. 5,347,121.

BACKGROUND OF THE INVENTION

The field of the present invention relates to data reading systems andparticularly to an optical system having an expanded depth of field. Theinvention is especially suitable for use with a symbol scanning systemfor reading bar codes such as those found on consumer products, forexample the UPC code. The invention is suitable for stationary orhandheld scanners.

Bar code scanners, as any optical system, depend upon focused optics foreffective and accurate performance. Typical bar code scanners employ asource of coherent light from a laser or laser diode with the lightscanned in various directions across a window. Other scanners usingnon-coherent light sources have also been suggested such as disclosed inU.S. Pat. No. 4,335,302.

In a detection system such as a bar code scanning device employing afocusing lens, a light source such as a laser, laser diode, ornon-coherent light source (e.g. light emitting diode) emits light whichpasses through and is focused by the focusing lens. The objectcontaining the bar code is passed through the focused beam and if thebar code is sufficiently close to the beam focal point, reflected lightfrom the bar code may be detected resulting in a successful scan.

As known by one skilled in the art, a focal point is typically not adiscrete point but may be referred to as a "waist" which is the positionalong the beam axis where the "cone" of light from the light sourcereaches a minimum spot size, usually as measured in a direction parallelto the direction of spot motion.

A problem arises when the bar code or object being scanned does not fallsufficiently close to the focal point or waist, that is when the beamspot is too large to successively read a symbol. By way of example, in asupermarket checkout application, a product bearing a UPC bar code labelis passed at a certain distance in front of the window of a checkoutscanner. The checkout scanner is designed with a scanning beam with awaist of a given diameter positioned at a certain distance from thewindow where the bar code is expected to pass. The checkout clerk mustbecome familiar with the proper distance to pass the object in front ofthe window, that is, the bar code must pass sufficiently close to thescanner focal point or waist (i.e. within its depth of field) in orderto achieve a successful scan.

However, in some applications, it may be desirable for the scanningdevice to function over a range of distances. There have been severalsuggestions on how to increase the depth of field or selectively choosea depth of field available for a particular scanner. In one system, afocusing lens is designed with an axially movable lens element (such asa zoom lens) to permit changing of the position of the focal point. Suchsystems require complicated mechanical lens adjustment and/or mayrequire the user to manually make focusing adjustments.

U.S. Pat. No. 4,808,804 discloses mechanical mechanisms for varying theworking distance and the beam spot size. In systems as disclosed in U.S.Pat. No. 4,818,886, the position of the detector or the light sourceitself is moved changing the object distance. It is desirable toeliminate the need for focus adjustment either by mirror, lens, orsource position adjustment and be able to achieve a wide range orvariable range of waist location.

Another attempt at providing multiple depths of field is described inU.S. Pat. No. 4,560,862 which uses a rotatable optical polygon mirrorhaving a plurality of facets, each mirror facet being of a differentcurvature. As the polygon mirror rotates, a different mirror facetreflects the beam from the light source along an optical path, eachmirror facet providing a corresponding focal plane. The device requiresmultiplexing the signal to read the signal received from the variousfocal planes. Since the rotating polygon mirror also scans the outgoingbeam, the device may also not be readily compatible with existingscanner designs and only allows a certain number of discrete focalpoints (one focal point for each mirror facet). Moreover, changingbetween selected sets of focal points would require replacing mirrorfacets or making some other complicated hardware adjustment ormodification.

SUMMARY OF THE INVENTION

The present invention relates to an optical system and method for datareading. The system includes (1) a light source which generates anoptical beam directed toward an object, (2) a focusing system and (3) avariable aperture optical element disposed in the outgoing optical path.The aperture size is smaller than the diffractive limit of the opticalbeam. Being smaller than the diffractive limit means that the apertureimpinges on the optical beam enough so as to affect the beam propagationin accordance with diffraction theory. The waist position is thenchanged by varying the effective aperture size. In a preferredembodiment, the variable aperture optical element is located downstreamof the focusing lens and comprises a plurality of liquid crystal devicessuch as an LCD (Liquid Crystal Display) panels disposed along onedimension forming a desired gate width. As the liquid crystal devices orpanels are selectively activated, the width of the gate iscorrespondingly increased or decreased. When the light beam is focusedto a waist at a given distance from the focusing lens, this waist may bemoved closer to the focusing lens by decreasing the gate width size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an optical system accordingto the present invention;

FIG. 2 is a diagrammatic top view of a portion of the optical system ofFIG. 1 illustrating an LCD embodiment;

FIG. 3 is a detailed front elevation view of the LCD gate element ofFIG. 2;

FIG. 4 is a detailed scale view of the LCD gate element of FIG. 3showing preferred dimensions;

FIG. 5 is a cross sectional view of the LCD gate element of FIG. 4 takenalong line 5--5;

FIG. 6 is a detailed scale view of an alternate LCD gate element showingpreferred dimensions;

FIG. 7 is a diagrammatic view of alternate LCD gate element of a singlerectangular panel pair;

FIG. 8 is a diagrammatic view of alternate LCD element of a round irisconfiguration;

FIG. 9A is a diagrammatic view of alternate aperture element comprisedof a rotating aperture;

FIG. 9B is a top plan view of the aperture element of FIG. 9A;

FIG. 9C is a cross sectional view of the aperture element of FIG. 9Ataken along line 9C--9C;

FIG. 10 is a diagrammatic view of alternate mechanical aperture elementof a round iris configuration;

FIG. 11 is a diagrammatic view of alternate mechanical aperture elementcomprised of a pivoting shutter configuration;

FIG. 12 is a diagrammatic view of alternate mechanical aperture elementcomprised of a sliding shutter configuration;

FIG. 13 a graph illustrating plots of example spot diameters for anexample multi-width variable focus LCD gate element;

FIG. 14 a graph illustrating the shift in waist position as the LCDdrive voltage is varied which changes the grey-scale;

FIG. 15 diagrammatically illustrates a preferred control method fordriving the LCD module;

FIG. 16 a graph illustrating the frequency dependence of LCDgrey-scaling for different temperatures;

FIG. 17 is a schematic diagram of an alternate polarization adjustmentsystem;

FIG. 18 is a graph of a beam profile of a first sample visible laserdiode module;

FIG. 19 is a graph of a beam profile of a second sample visible laserdiode module;

FIG. 20 is a graph of a beam profile of a third sample visible laserdiode module;

FIG. 21 is a diagrammatic front view of an LCD gate mechanism with aconcentrically positioned opening;

FIG. 22 is a diagrammatic rear view of an LCD gate mechanism of FIG. 21from the light source side;

FIG. 23 is a diagrammatic front view of an LCD gate mechanism with anoffset opening;

FIG. 24 is a diagrammatic rear view of an LCD gate mechanism of FIG. 23from the light source side;

FIG. 25 is a schematic drawing of a preferred integral laser diode andfocusing system;

FIG. 26 is a graph illustrating waist diameter as a function of aperturewidth;

FIG. 27 is a graph illustrating a comparison of waist relocation of asample beam apertured by an LCD gate device;

FIG. 28 is a flow chart representing a preferred focus adjustmentmethod;

FIG. 29 diagrammatically illustrates an aperture element with a jaggededge.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiments will now be described with reference to thedrawings. To facilitate description, any numeral identifying an elementin one figure will represent the same element in any other figure.

FIG. 1 is a schematic diagram of an optical system such as that whichmay be employed by the present invention. A light source illustrated asa laser diode 10 emits light 15 being aimed at the desired target shownas the UPC bar code 35. Light 15 from the laser diode 10 is passedthrough focusing optics, illustrated in this embodiment as a focusinglens 20. The focused beam 15 is then passed through an aperture device50. The portion of the beam 15 which passes through the aperture device50 is reflected off a fold mirror 25 and is then directed toward ascanning mechanism shown in this embodiment as an oscillating mirror 30.As the oscillating mirror 30 pivots, the beam scans over a scanning beamrange 32 along the bar code 35.

Light reflected or scattered off the bar code 35 is collected by asuitable collection system shown in this embodiment as being focused bya collection lens 40 and detected by the photodetector 45. The optics ofthe optical system are constructed such that the focusing lens 20achieves a waist at a distance from the system at or near theanticipated furthest position of the targeted bar code 35.

Though a preferred scanning mechanism and collection system have beendescribed, any suitable scanning mechanism or collection system may beutilized in the present optical system. As for light sources, the lightsource 10 is preferably a laser diode, but may be any suitable lightsource including: a coherent light source such as a laser or laserdiode, a non-coherent light source such as a light emitting diode, orcombinations thereof. The focusing system may be comprised of one ormore optical elements selected from the group consisting of: spherical,Fresnel and aspheric lenses or mirrors, holographic optical elements,and combinations thereof.

To clarify terminology, as used herein when referring to a scanner, theresolving axis shall refer to the axis of the spot along the scanningdirection. The non-resolving axis shall refer to the directionperpendicular to the scanning direction.

FIGS. 1-2 illustrate the aperture device 50 positioned downstream of thefocusing lens 20 between the focusing lens 20 and the fold mirror 25.The aperture device 50 is preferably positioned downstream of thefocusing lens 20 so that the beam 15 is converging as it passes throughthe aperture. The variable aperture device may however be located atother positions such as between the light source 10 and the focusinglens 20 or between the fold mirror 25 and the scanning mirror 30. If theaperture device is to be positioned on the light source side of thefocusing lens 20, it is preferred that the device be located adjacentthe focusing lens (as opposed to near the source 10).

Essentially, when control of the spot size in the resolving axis isdesired (control of the waist position in the non-resolving axis isdescribed below) the aperture mechanism 50 is preferably on the lightsource side of the scanning mirror 30. Alternately, the variableaperture mechanism 50 may be incorporated into the scanning mirror 30itself, oscillating therewith.

In the configuration where the aperture mechanism were positioned on thetarget side of the scanning mirror, the shape of the beam could bemanipulated for different parts of the scan, the aperture mechanismhaving a more complex structure, for example a series of apertures.

FIGS. 2-5 illustrate a preferred construction for the aperture mechanism50. The aperture mechanism 50 has a rectangular central aperture 52which is a light transmissive element such as clear glass. The width ofthe aperture 52 is arranged parallel to the scanning plane of thescanning beam 32, that is, in the resolving axis (referring to FIG. 2,the beam would scan in a plane parallel to the page). The aperturemechanism 50 has a first pair of transmissive LCD elements 54, 54 (theLCD pair comprising an LCD region which defines an aperturetherebetween) symmetrically positioned on either side of the centralaperture 52 and a second pair of transmissive LCD elements 56, 56 oneither side of the first pair of transmissive LCD elements 54, 54. Bothsets of LCD elements 54, 56 are mounted on a suitable glass substrate51. In some cases, as described below, the aperture may be non-symmetricor offset, placed in such a way as to block one side of the beam morethan the other. This offset arrangement may minimize sidelobes in thebeam profile that occur as a result of near-field diffraction as will bedescribed in more detail below.

When an LCD is not energized, the polarization of light passing throughit is rotated 90° . A polarizer 53 is placed beyond the LCD and isoriented 90° to the initial light polarization. This construction allowsthe light to pass through with minimum absorption. The polarizer may bea sheet polarizer (a sheet of polarizing material), a polarizing beamsplitter, a polarization-dependent mirror, or any other suitable opticaldevice that keeps light polarized along one plane from continuingthrough the optical device while allowing light polarized along theorthogonal plane to continue through the optical device. It is nonrequired that the polarizer be in close vicinity to the liquid crystaldevice, however it must be on the side of the LCD opposite to the lightsource. For example, the polarizer may be incorporated into the foldmirror 25 or the scanning mirror 30. It is conceivable that thepolarizer may be incorporated into rotating optical polygon scanningmirror, but such a configuration would not be preferred. In onepreferred embodiment, the fold mirror 25 may comprise a polarizing beamsplitter (incorporating the polarizer 53 therein) in which the polarized(aperture) light is reflected and the remainder beam portion 17 passesthrough the element 25 to an alternate device 27. The remainder beamportion 17 may serve an alternate function such as an aiming beam, asecondary scanning beam, a timing beam, an illuminator beam, or afeedback signal beam, or some other suitable function.

When the LCD is energized, the light polarization is no longer rotated.If polarized light is being passed through the LCD panels, the lightpolarization is rotated by 90° back to the original polarization whenthe LCD is energized which in combination with the downstream polarizerinhibits the passage of light therethrough. In the application where thelight source 10 is a polarized laser or laser diode, the light impactingthe LCD is already about 99% polarized. The LCD regions are thenarranged to inhibit passage of light therethrough when energized. In theapplication where a non-coherent light source is used (or to furtherensure desired polarization), a polarizer is also placed upstream of theLCD module and the areas where the LCD is activated, which incombination with the downstream polarizer will not allow light through.

The net effect of the aperture mechanism 50 is to have three separatefoci A₁, A₂, and A₃ formed from a single axis of light merely byactivating the respective pair of LCD regions or panels 54, 56 of theaperture mechanism 50. Specifically, when none of the pairs of LCDregions 54, 56 are energized, the width of the effective aperture is Aand the focus waist will then appear at point A₁. By energizing theouter LCD panels 56, 56 (preferably simultaneously), the width of theeffective aperture is reduced to a dimension B. Since the dimension B iswithin (i.e. smaller than) the diffractive limit of the beam 15, thefocus waist will now appear at point A₂. Subsequently, by alsoenergizing the inner LCD panels 54, 54 and the outer LCD panels 56, 56,the width of the effective aperture is further reduced to a dimension Cand the focus waist will now appear at point A₃.

The following are a set of preferred dimensional values for the elementsof aperture mechanism 50 set forth in FIGS. 4-5:

A=0.200 in. (5.1 mm)--the overall aperture outside of the LCD panels 56,56.

B=0.070 in. (1.8 mm)--the aperture defined between the LCD panels 56,56.

C=0.035 in. (0.89 mm)--the aperture defined between the LCD panels 54,54.

D=0.17 in. (4.3 mm)--the height of the active LCD cell.

E=0.043 in. (1.1 mm)--the thickness of the glass substrate 51.

F=0.086 in. (2.2 mm)--the total thickness of the aperture 50.

In data reading applications such as bar code scanning systems, thisdesign provides several advantages. A system with three discrete waistpositions A₁, A₂, and A₃ would have three different depths of fieldregions (one for each focal position) resulting in an overall greaterdepth of field than a conventional system with only a single focalposition. The system is electrically controllable with no moving parts.Since the LCD regions are readily actuated, such a design is simple andrelatively inexpensive and does not require complicated movable focusingelements.

The specific design of the variable aperture mechanism 50 will depend onvarious factors for a particular application including the number ofwaist positions desired, the type of light source, light sourceintensity distribution, target size and type, and external factors suchas the desired distances to the focal points, spot diameters at thewaist positions, aperture stops included outside of the lens system,lens diameter(s), and cost constraints. The example of FIGS. 3-4 ismerely a preferred example which may be particularly useful in ahandheld bar code scanner. In one example tested (using a mechanicalaperture of a construction similar to the one in FIG. 12 describedbelow), without the variable aperture mechanism 50, the unmodifiedscanner would have the capability of reading a 13 mil (330 micron)labels over a range of 30-60 inches (760-1520 mm) whereas with thevariable aperture mechanism 50, the scanner could effectively read overa much broader range of 6-60 inches (150-1520 mm).

At first glance it may appear problematic that the addition of anaperture in the light path operates to reduce total light intensity.However, when the aperturing is effected downstream of the focusing lens20 (referring to FIGS. 1-2) and further than one focal length from thefocusing lens, the addition of the aperture moves the waist closer tothe focusing lens. Therefore, the light intensity loss due to aperturereduction is compensated for by the nearer waist location actuated bythe smaller aperture. The compensation is due to the fact that lightintensity varies in proportion to 1/x² (x being the distance from thelight source).

If on the other hand the aperturing occurs on the light source side ofthe focusing lens 20 (i.e. upstream of the focusing lens 20), theaddition of an aperture moves the object location further away from thefocusing lens. Therefore the aperture not only reduces the lightintensity, but the focus distance increase compounds the loss of lightintensity. It is therefore preferred that the effective aperture occurdownstream of the focusing lens or if located upstream of the focusinglens, it is preferred that the aperture be located at least in closeproximity thereto, preferably less than one focal length from thefocusing lens. By placing the aperture within one focal length of thelens, no image (of the aperture) is formed. In contrast if the aperturewere placed greater than one focal length upstream of the focusing lens,an image of the aperture would be formed and reducing aperture sizewould shift the image further from the focusing lens. Therefore it ispreferred that the aperture 50 be located downstream from a point onefocal length upstream of the focusing lens 20. In other words, for alens 20 having a focal point one focal length upstream (i.e. to the leftas viewed in FIG. 2) thereto, it is preferred that the aperture 50 belocated downstream (i.e. to the right) of that point but preferablyupstream of the scanning mirror 30.

FIG. 25 illustrates a scanner 560 of preferred integral construction inwhich the liquid crystal element 562 is incorporated into the housing orbarrel 565 of the light source shown as a laser diode 564. The beam oflaser light from the laser diode 564 is focused by a focusing lens 566and then is passed through the liquid crystal element 562. In thisembodiment, the polarizer is not located adjacent to the liquid crystalelement but well downstream at a polarizing beam splitter orpolarization dependent mirror 570. Tests have shown that with thedownstream polarizer removed, the beam profile is unaffected by theliquid crystal element regardless of whether it is activated or not. Theeffective aperturing takes place at the polarizer 570. Thisconfiguration enables the scanner 560 to be readily constructed with theliquid crystal element 562, focusing lens 566, and laser diode module564 assembled within the same housing 565 in a unitary structure. Thestructure may be assembled by the laser diode manufacturer and sealedwithin the barrel housing 565 providing pre-alignment for the elementsand a protective outer structure to safeguard the elements from damageor misalignment during scanner assembly or use. The liquid crystalelement 562 may be positioned on either side of the focusing lens 566 asit is the location of the polarizer 570 relative to the focusing lens566 which is significant.

Though multiple waist positions are often desirable, in certain otherconfigurations, only a single waist position may be desired. Theaperture mechanism 50 may provide a preset focal point, set by themanufacturer, the system assembler, or the technician (or alternatelythe user by actuation of an external switch). In such a system, a singlescanner assembly design may be employed and the manufacturer (forexample) need only select the appropriate aperture setting correspondingto the desired fixed focal position. Manufacturing costs may be reducedas only a single scanner design need be manufactured without requiringcomplicated hardware modifications. Further, by allowing easy adjustmentof the focal position, the required mechanical tolerances of the otheroptics may be reduced.

In order to have separately addressable LCD regions, in currenttechnology, a non-active region is required between the LCD regions. Asviewed in FIGS. 3-4, the non-active regions 58, 58 (shown only as a darklines 58, 58 in FIG. 4) separate the inner LCD panels 54, 54 from theouter LCD panels 56, 56. These non-active regions have not shown tocreate any significant impact on aperturing effects.

FIG. 6 illustrates an alternate LCD aperture element 150 also showingpreferred dimensions for the inner LCD panels 154, 154, the outer LCDpanels 156, 156, and the central aperture 152. In order to ensureseparately addressable LCD regions and to provide for easierconstruction, a larger non-active region is required between the innerLCD panel regions 154, 154 and the outer LCD panel regions 156, 156. Thenon-active regions 58, 58 (shown as the cross-hatched strips 158, 158)separate the inner LCD panels 154, 154 from the outer LCD panels 156,156. The preferred dimensions are as follows:

G=0.200 in. (5.1 mm)--the overall aperture outside of the LCD panels156, 156.

H=0.070 in. (1.8 mm)--the aperture defined between the LCD panels 156,156 inside of non-active regions 158, 158.

I=0.005 in. (0.13 mm)--the thickness of the non-active region 158.

J=0.035 in. (0.89 mm)--the aperture defined between the LCD panels 54,54.

K=0.17 in. (4.3 mm)--the height of the active LCD cell.

FIG. 13 is a graph of experimental data showing optical performance ofone example embodiment of the optical system employing a mechanicalaperture which applied the aperture dimensions of the aperture mechanism50 of FIGS. 2-5. The x-axis shows where a value on the curve iscalculated as the distance from the lens in mm. (This is the distancebetween, for example, aperture mechanism 50 and point A₃ in the shortfocus case of FIG. 2.) The y-axis represents the spot diameter in theresolving axis of a laser beam focused by the system. This spot diameteris expressed in microns (units of 10⁻³ mm) and is measured at the widthof the focused laser beam where the intensity is 1/e² times as large asat the center of the focused beam.

The graph of FIG. 13 shows three different curves of data. The solidline Curve E represents the change in spot diameter for a laser beamwith a wavelength of 670 nm, with a best focus diameter (in theresolving axis) of about 470 microns, the near focal point A₃ (when bothLCD panels 54, 56 are energized) is located about 300 mm from theaperture. On either side of the best focus, the spot diameter getslarger, as shown in the solid curve.

The dashed line Curve F represents the intermediate focus point A₂created by energizing only the outer LCD panels 56. In this case thefocus point A₂ has been positioned at about 560 mm and has a minimumspot diameter in the resolving axis of about 560 microns. The dottedline Curve G represents the far focus point A₁ created with neither LCDpanel 54, 56 energized. In this case, the focus point A₁ has beenpositioned at 850 mm and has a minimum spot diameter in the resolvingaxis of 480 microns.

To illustrate how this system would extend depth-of-field over aconventional single focus system, an example signal processing systemwill now be considered. In one kind of signal processing, it may bepossible to successfully "decode" signals scanned where the 1/e² spotdiameter of the spot is 2.0x as large as the minimum bar width beingscanned. Using this assumption as a guideline, a spot 700 microns indiameter would be able to read bar codes with a bar width of 700/2.0microns, which is 350 microns. If the spot size is 700 microns orsmaller, then, it is possible under this signal processing assumptionthat a scanner could "decode" labels of 350 microns or larger.

Now, drawing a line across curves E, F and G at the 700 micron spot sizepoint, it may be seen that the short focus aperture setting lens elementwill be able to resolve the 350 micron bar widths over a distance ofabout 340 mm, or from 160 mm (at the closest point) to a distance of 500mm (the furthest point). The intermediate aperture setting will be ableto resolve the 350 micron bar widths over a distance of about 300 mm, orfrom 500 mm to 800 mm. The far aperture setting will be able to resolvethe 350 micron bar with this over a distance of about 440 mm, or from620 mm to 1060 mm. Thus by cycling through the three aperture settings,the device may decode the labels of 350 microns or larger from 160 mm to1060 mm.

FIG. 7 illustrates another alternate LCD aperture mechanism 250 having asingle set of LCD panels 256a, 256b defining a central aperture 252. Thefirst LCD panel 256a is connected to a controller (see FIG. 15 below) byconnection elements 257a and 257b. The second LCD panel 256b is alsoconnected to the controller by connection elements 257c and 257d. Theoperation of the LCD aperture mechanism 250 is similar that described ofprevious LCD aperture embodiments. The aperture mechanism 250 controlsonly a single aperture region by LCD panels 256a, 256b, preferableactivated in unison. This embodiment may be particularly applicable togrey-scaling techniques, described below, to provide a continuouslyvariable or controllable waist position.

Typically it might be expected to place the LCD gate symmetrically atthe center of the beam path. The present inventor has recognized that anoffset or non-symmetric location may actually be preferred. In many ofthe laser diodes tested, when aperturing the parallel (low divergence)axis of the beam produced by a laser diode, there is little change tothe beam profile as the opening of the gate element is moved across thebeam. However, when aperturing the perpendicular (high divergence) axisof the beam produced by a laser diode, the location of the gate openingproduces significant changes in the beam profile at the beam waist.Tests have shown that there is significant variation in beam profile oflaser diodes even of the same model and specifications.

FIGS. 18-20 are graphs of beam profiles for sample visible laser diodesas measured at a near field 10 inches (25.4 cm) from the light source.In FIG. 18, the beam profile P₁ is shown with the intensity of the beamhaving a higher value on a left edge thereof. In FIG. 19, the beamprofile P₂ is shown with the intensity of the beam having a highervalues on both a left edge and a right edge thereof. In FIG. 20, thebeam profile P₃ is shown with the intensity of the beam having a highervalue on a right edge thereof. The present inventor determined fromthese beam profiles, P₁, P₂, P₃ that the location of the gate mechanism,being either centered or offset to one side, may alter performance ofthe waist location shift effected by the gate mechanism.

FIG. 21 illustrates a gate element 510 from a position looking towardthe light source and FIG. 22 illustrates the gate element 510 from thelight source side. The LCD gate element 510 is located with its aperture515 concentrically positioned with the beam 520. Depending on theparticular beam profile, it may be desirable to locate the gatemechanism offset to the central axis of the beam. FIGS. 23 and 24illustrate an offset LCD gate element 530 where the opening 535 islocated offset from the central axis of the beam 540, i.e., toward aside edge of the beam 540 the right side as viewed from the light sourceside.

An offset gate mechanism having its opening offset to one side (say forexample the right side as shown in FIG. 24 as viewed from the lightsource side) of the beam may achieve superior waist adjustment for afirst diode tested, but a second diode with a different beam profile maynot achieve the same superior results and in fact by locating theopening offset to the opposite side of the beam, superior results may beachieved. It therefore may be desirable to provide specifications forthe light source in order to obtain specific beam profiles to ensuresuperior results when offsetting the gate mechanism to a given side ofthe beam. However, it is possible that a laser diode manufacturer willnot be receptive to such specifications or that the cost of complyingwith the specifications may be prohibitive.

To conform the laser diode to the optimize multifocal performance it isdesirable to understand the mechanism by which the laser diode beam isapertured. It has been determined that the laser diodes may beclassified into certain groups of beam profiles. For example, ifsubstantially all of a manufacturer's laser diodes varied between thethree profiles of FIGS. 18-20, an assembly may be employed to compensatefor variation in beam profile. In the case of a beam profile P₃ of FIG.20 (as viewed from the light source side) with a spike to the right handside, it is preferred that the gate opening 535 be offset to the rightas shown in FIG. 24. In the case of a beam profile P₂ of FIG. 18 (asviewed from the light source side), it is preferred that the gateopening 535 be offset to the left. When assembling the scannercomponents, the LCD gate may be installed with the opening set to thedesired side. For example, the gate mechanism may be sidewardlyadjustable to position the gate opening at the desired location.Alternately, the gate mechanism having a built-in offset opening may berotated 180° to reposition the opening to the left or right side (thepolarizer would need be located downstream. In a preferred design, thegate mechanism is positioned with a fixed offset and the system isadjusted by rotating the laser diode module 180° to reposition the beamrelative to the offset opening. It is preferred that the laser diode andthe focusing lens optics be mounted together in the barrel housing androtated together as a unit.

A suitable testing mechanism may be employed to determine the preferredoffset orientation for a particular laser diode. In one such system, thelaser diode to be tested is placed in a test fixture with the laser beamthen scanned across a series of bars on a clear plastic plate havingseries of detectors thereon, the plate being located at 22 inches (56cm) from the laser diode. The scanning beam is tested by passing thebeam through an aperture mechanism, first with the opening offset to oneside (with a measurement taken) and second with the opening offset tothe other side (with a measurement taken). The first and secondmeasurements are then compared to determine the preferred orientationfor the laser diode relative to the offset of the opening. The diode maythen be suitably tagged or marked. During assembly, the laser diode isinstalled (relative to the location of the offset) according to the tagor marking so the diode is positioned with its preferred orientation.

Following the theory of Gaussian Beam Propagation, the waist diameter isinversely proportional to the beam diameter of the lens. By aperturingthe beam at the lens, the effective beam diameter is reduced and theresultant waist diameter is increased. This relation is not true whenthe distance to the waist is also changed. FIG. 26 shows a graph of thewaist diameter as a function of aperture width. Curve H is a plot of theillustrates the result of the gate opening being concentric or in themiddle of the beam. Curve I illustrates the result of the gate openingbeing offset to the right side of the beam and Curve J illustrates theresult of the gate opening being offset to the left side of the beam. Inthis case a visible laser diode is being measured along theperpendicular axis. This definition represents the axis which has alarger divergence angle. The perpendicular axis has the peculiarcharacteristic that the sidelobes are greatly reduced when the beam isapertured from one side or the other rather than in the center. Theintroduction of sidelobes makes precise definition of the waist sizedifficult. The graphs H, I, J indicate that in all three cases, thewaist diameter does increase as the aperture width is decreased. In theedge aperture cases (I, J) this trend is reversed as the width isdecreased to 80%-90% of the beam diameter at the lens. From this point,the waist diameter decreases as a function of the aperture width untilit levels off for small apertures. The centered aperture plot H showsthat the waist diameter continues to increase until 50% of the beam iscut off by the aperture. After this point, it also exhibits a reductionin the waist diameter as a function of the aperture width.

As the aperture width is reduced, the waist location shifts toward thelens because of diffraction effects. FIG. 27 is a graph of the waistlocation as a function of the aperture width comparing waist relocationof a sample beam apertured by an LCD gate device. Curve T illustratesthe result of the gate opening being concentric or in the middle of thebeam. Curve R illustrates the result of the gate opening being offset tothe right side of the beam and Curve S illustrates the result of thegate opening being offset to the left side of the beam. The graphillustrates the superior aperturing performance of the gate openingbeing located on the left side of the beam providing a generally lineardecrease in distance from the source to the waist location. The data inthis graph corresponds to the waist diameter data in FIG. 26. The edgeapertures work much better than the centered aperture for translatingthe width. When the centered aperture width is reduced, the distancedoes not change by very much until the aperture is on the order of 50%of the lens diameter. This phenomena is partly due to the sidelobeswhich are introduced. It may be noted that the turning point in waistdiameter corresponds to the point where the waist begins to shift towardthe lens.

Alternate aperture mechanisms may be designed for given applications. InFIGS. 2-5, the aperture mechanism 50 is configured from a plurality oflongitudinal LCD panels 54, 56 with the width of the aperture(corresponding to the resolving axis) being incrementally varied byselective activation of the LCD panels 54, 56. The LCD regions mayalternately be rectangular as shown in the previous embodiments or mayalternately be circular, oval or any desired geometry or configurationto vary the waist location in two dimensions.

FIG. 8 illustrates such an alternate geometry for the aperturecomprising LCD aperture mechanism 350 having a single circular LCD panel356 defining a central aperture 352. The LCD panel 356 is connected to acontroller (see FIG. 15 below) by suitable connection elements. Byforming a round (or any other suitable shape) aperture 352, the spotsize (i.e. at a given waist position) is controlled in both axes. Forexample as applied to a 2-D bar code, control of the focal pointlocation would be provided in both the resolving and the non-resolvingaxis. The operation of the LCD aperture mechanism 350 is similar to thatdescribed of previous LCD aperture embodiments. This embodiment may beparticularly applicable to grey-scaling techniques, described below, toprovide a continuously variable or controllable waist position. Theaperture mechanism 350 controls only a single aperture region by LCDregion 356, but may alternately be comprised of a second or morecircular LCD regions concentrically positioned relative to the firstregion.

Though the aperture regions may be incrementally activated to obtaindiscrete waist positions (discrete meaning that the change in waistposition is incremental--similar to a step function), the LCD regionsmay be partially activated to block some, but not all of the lightpassing therethrough. Tests have shown that the beam waist position maybe moved to any intermediate position between the near and far waistlocations as defined by an inactive and fully active LCD apertureregion. Using the methods similar to those used by portable computermanufacturers to generate continuously variable grey-scale on a liquidcrystal display screen (by for example varying the applied voltage), theLCD aperture may be adjusted to a desired grey value thereby "setting"the beam waist at any selected intermediate location.

FIG. 14 is a graph illustrating results of a test where the shift inwaist position by grey-scaling is controlled by varying the drivevoltage applied on an LCD aperture device 250 such as that shown in FIG.7. As the voltage applied to the LCD is varied, the percent activationof the LCD is adjusted from 0% to 100%. As shown in the graph, byincreasing the voltage applied to the LCD from 0 to 2.0 volts, the waistlocation is moved from 910 mm to 410 mm from the aperture.

The grey-scale, continuously variable aperture technique thereforeprovides a continuously adjustable (or selectable) waist position. Byselecting a given LCD activation level (such as by applying a givenvoltage to the LCD), any desired waist position within the range may beachieved. Such a continuous focus system has a variety of applications.One highly desirable application is automatic focus provided some sortof feedback is available. Distance measurement techniques such as anoptical or ultrasonic (such as those employed by autofocus cameras) isone focus feedback method, but other methods may be employed such asfocus error techniques may be employed. For example, optical sensors inthe beam path may determine the scan distance and provide a signal tothe aperture controller which may be used to select the desired apertureprovided by the variable aperture mechanism. Alternately, the scanreturn signal may be observed (analog or digital) and then determinedwhether the symbol (e.g. the bar code) is being accurately reproduced.By sweeping through the range of available waist positions, it can bedetermined at what focal position provides the most successful scanningoperation. A preferred adjustment algorithm is described below withreference to FIG. 28.

This waist location shift is a diffractive phenomena. The aperture sizeis smaller than the diffractive limit of the optical beam, i.e., theaperture impinges on the optical beam, enough so as to affect the beampropagation in accordance with diffraction theory. The phase and spatialintensity of the light wavefront at the variable aperture device istherefore being modified. In the case of the LCD, the spatial intensityis being modified. This phenomena can be used to alter the beam profileat a given location as well as move the waist location. Alternately,materials that change the optical phase of the wavefront when electricalor optical signals are applied to them (i.e. non-linear opticalmaterials) may be used in place of the LCD. In these cases, selectiveregions may be activated and the wavefront phase be modified. Thisalternate structure may be designed to alter the waist location orchange the beam profiles to allow improved performance. These materialsare currently expensive but may prove economically viable in the future.

The LCD can be designed to form a wide variety of spatial intensityprofiles. This technique can be extended to the situation of creating anElectronic Holographic Optical Element or Binary Optical Element. Thiselement is an optical element that has no curvature, but has regionsthat are opaque which form a diffraction pattern that has an intensityprofile suitable for resolving the bar code.

The edge of the aperture element may be designed to diffractively limitsidelobes in the beam profile by providing an edge which is irregular ornot straight. FIG. 29 illustrates one possible construction of anon-straight, jagged edge for an aperture element 670 comprised of firstset of liquid crystal panels 674, 676 defining an offset aperture 673.The first liquid crystal panel 674 is connected to a controller byconnection elements 682 and 684. The second liquid crystal panel 676 isalso connected to the controller by connection elements 682 and 684. Theoperation of the liquid crystal mechanism 670 is similar that describedof previous LCD aperture embodiments. The non-straight edges 675, 677 ofthe elements 674, 676 allows interference between the edge contributorswhich may reduce sidelobes of the beam profile and improve performance.The aperture element 670 provides a second narrower width 672 byactivating inner liquid crystal panel 678, the inner liquid crystalpanel 676 is also being connected to the controller by connectionelements 686 and 688. The inner liquid crystal panel also includes ajagged edge 679.

FIG. 15 diagrammatically illustrates a preferred control method used todrive the LCD module (such as the variable aperture mechanism 50 ofFIGS. 4-5) with a microprocessor 70 that is already present in thescanner. The microprocessor 70 signals the controller 75 (the two may becombined into a single microprocessor unit 80) to activate the LCDregions to the desired intensity. The microprocessor 70 is used toperform operations on the digital signal received from the detector 85and prepare it to be sent to the reader 90 (such as a Portable DataTerminal which decodes the signal and sends it to a host computer).Since information such as wide-to-narrow ratios and number of digitaltransitions are calculated by the microprocessor 70, the information canbe used to determine if the beam size is small enough (relative to thebar code size, for example) to resolve the smallest bars and spaces. Themicroprocessor 70 uses this information as feedback to adjust the waistlocation by increasing or decreasing the LCD activation level. Given theteachings herein, one skilled in the art could develop suitable softwareto optimize the system for a given application.

The response time of the liquid crystal material may be highly dependentupon temperature. For example, if the response time of the liquidcrystal material is 10 msec at 20° C., it will increase to 100 msec at-20° C. The aperture mechanism may therefore be provided with atemperature measurement device shown in FIGS. 4-5 as a thermistor trace60 placed on the glass substrate 51 adjacent the LCD panels 54, 56. Thethermistor can also be placed elsewhere in the scanner as long as theambient temperature is similar to that of the LCD. The system may thenbe controlled by monitoring the temperature and altering the operationfor different temperatures. Referring again to FIG. 15, a signal fromthe thermistor 60 may be digitized and read by the microprocessor 70.The microprocessor 70 receives information from thermistor 60, andthrough suitable applications software, may control the way the LCDregion of the aperture mechanism 50 is activated as a function of thetemperature. For example, when sweeping or cycling across the focalrange, the LCD regions may be activated for a longer period of time ifthe temperature is lower in order to accommodate for the increase LCDrise time.

Even without distinctly varying the LCD regions, the inherent rise andfall time for the LCD regions may allow for a sweep of LCD intensityfrom minimum to maximum. During the transition time, several scans maytake place while the LCD intensity rises. During the change ofintensity, grey-scaling occurs varying the beam waist from between themaximum and minimum distances.

To avoid temperature variation, the system may be provided with aheating element 65 in proximity to the aperture mechanism 50. Theheating element may be controlled by a suitable thermostat, such as bycontroller 75 on the basis of the temperature detected by the thermistor60.

In some scanners, the existing microprocessor may already have an E²PROM available for use. This E² PROM may also be used as a look-up tablefor LCD activation parameters in the temperature compensation controlscheme or for some grey-scale control. For example, the voltage levelrequired to establish a certain grey-scale will vary from unit to unitand over the operating temperature range. Discrete values for thevoltage can be measured and input during the scanner manufacturingprocess and written into memory. For a given measured operatingtemperature, the look-up table would provide the information of avoltage correction which would compensate for temperature. Thisprocedure may greatly simplify the electronics and improve performanceover temperature range.

Grey-scaling of the LCD regions may also be accomplished by driving theLCD regions at frequencies much higher than the normal operatingfrequency. FIG. 16 shows experimental data for one sample. As thefrequency increases, the liquid crystal molecule alignment becomes lesspronounced and the region blocks less light. As the temperatureincreases, the frequency limit also increases. In FIG. 16, curve K shownby a solid line illustrates how the transmission varies with frequencyat a temperature of -20° C. Curve L shown by a dotted line illustrateshow the transmission varies with frequency at a temperature of 0° C.Curve M shown by a dashed line illustrates how the transmission varieswith frequency at a temperature of 20° C. Though it appears that afrequency controlled LCD is more sensitive to temperature fluctuationthan voltage control, frequency control may be viable option since themicroprocessor can readily change the drive frequency.

The voltage can be changed to establish a grey-scaling circuit by usingan RC circuit with a time constant below the expected processor outputfrequency (10 kHz-30 kHz). By varying the drive frequency with theprocessor, the AC signal that is passed through the LCD will also vary.A higher frequency will yield a lower peak-to-peak voltage across theLCD and produce the desired grey-scale effect. Separate electronics canbe used to convert the frequency or duty cycle output from thecontroller to a voltage level which controls the LCD drive voltage.

Alternately, the aperture size may be varied in the non-resolving axisto vary the waist location of the beam in the non-resolving axis(thereby controlling astigmatism). By controlling the beam waistposition with an aperture device in the non-resolving axis, the spotsize (at a given distance) in the direction perpendicular to the scandirection may be controlled. Depending upon the particular application,it may be desirable to control the spot size in the non-resolving axis,that is the axis perpendicular to the scanning plane. In certainsymbols, such as printed bar codes (such as dot matrix printed codes),the lines may contain voids and the spaces may contain specks. If thescanning beam was focused to a fine point in all directions, the scannermay detect the void as a space or the specks as a bar producing a falseread. For this reason, it may be desirable to produce an oval orelliptical shaped spot with a larger diameter in the non-resolving axis.The smaller diameter in the resolving axis allows resolution along thebars and spaces while the larger diameter in the non-resolving axisallows the spot being read to average out so that specks and voids donot cause a false read.

FIG. 28 is a flow chart illustrating a preferred LCD ActivationAlgorithm as it relates to an overall scanner system integration. Whenthe scanner is activated by trigger 610, the Last "good read" LCD focussetting 612 is read if it is available otherwise a default focusposition is chosen. The focus change controller 614 then determines,based on protocol, whether aperture temperature and settling timeadjustment controls are to be implemented. If the adjustment controlsare not to be implemented, the process skips to scanning step 626. Ifadjustment controls are implemented, the first adjustment step 616 isobtaining the temperature readout from the temperature device (such asthermistor 60 in FIG. 15). A grey scale controller 618 obtains anadjustment value from lookup table 620. Then the aperture settling times622 are analyzed and the aperture adjustor 624 makes the required focusadjustments to compensate for temperature and settling effects. Once theadjustments have been completed, the scanner 626 performs the scan(s)and collects the data. The decoder 628 attempts to decode the scandetected and the gate 630 determines whether or not there was asuccessful read. If the read was successful, the program proceeds toexit 642, if unsuccessful, the gate 638 determines whether or not theadaptive focus routine is to be implemented. If the answer is "NO",changer 640 signals the LCD to change to the next focus position insequence before returning to step 614. If the adaptive focus routine isimplemented, the analyzer 636 processes the scan data and if possibledetermines in step 634 whether or not a best focus position can beascertained. If best focus is determined, the setting sequencer 632 setsthe focus to the best focus position before returning to step 614. Ifbest focus cannot be determined, the changer 640 signals the LCD tochange to the next focus position in sequence before returning to step614.

Though the electrically actuable LCD aperture mechanism has manyadvantages, other aperture mechanism designs may be used. FIGS. 9A, 9Band 9C illustrate another alternate aperture comprising a rotatingaperture device 450 comprising a plate 454 with a central rectangularaperture 452. The plate 454 is mounted to a center post 457 that isperpendicular to the optical axis. As the post is rotated, theprojection of the aperture 452 along the optical axis becomes thinneruntil the effective clear aperture in the resolving axis is very small.In one test example with an aperture 452 having a width of 2.0 mm, thedistance of the waist from the laser module moved from 1100 mm to 245 mmas the aperture was rotated from 0° to 75°. The waist diameter (alongthe resolving axis) changed from 610 microns to 436 microns. The systemmay therefore be designed to read at a maximum distance. By adding asmall rotating aperture mechanism 450 in the beam path and selectivelyrotating the aperture 452 to only a few angle settings, the waist may bemoved from the furthest distance to other intermediate distances. Thescanner then sends the information to the decoder from all scandistances and the decoder then decodes whichever scans it can.

FIG. 10 illustrates another mechanical aperture comprising an aperturedevice 550 including a plate 554 with a round central aperture 552. Theaperture device 550 may be similar to the iris of a camera, movable toan aperture diameter of a desired size. Providing a round aperturegeometry, the iris type aperture device 550 will control waist locationin two dimensions. Such an iris device may alternately may be designedto provide an oval or other shaped aperture.

FIG. 11 illustrates another mechanical aperture comprising a dualshutter aperture device 650 including a first pivoting shutter element654a which oscillates on pivot rod 657a and second pivoting shutterelement 654b which oscillates on pivot rod 657b. In tandem, the shutterelements 654a and 654b form a variable aperture 650. By controlling themotion of the shutter elements 654a and 654b, such as actuated by amotor operably connected to the rods 657a and 657b, the size of theaperture 652 may be selectively varied. In the illustrated embodiment,the shutter elements 654a and 654b rotate in the same direction whenvarying the aperture size. Alternately, if the initial position (fromthat illustrated) of one of the shutter elements were rotated 90°, theshutter elements would be rotated in opposite directions.

FIG. 12 illustrates another mechanical aperture comprising a dual panelaperture device 750 including a first sliding panel element 754a and asecond sliding panel element 754b. The panel elements 754a and 754b movein tandem, both moving inwardly to reduce the size of the aperture 752or moving outwardly to increase the size of the aperture 752. In tandem,the shutter elements 754a and 754b form a variable aperture 752. Bycontrolling the motion of the shutter elements 654a and 654b, such asactuated by a motor mechanism operably connected to thereto, the size ofthe aperture 752 may be selectively varied.

Alternately, a system may include multiple aperture mechanisms,combining one or more of any of the above described aperture mechanisms(arranged in series for example) within a single system. For example, asystem may include two LCD aperture mechanisms (such as a pair of theaperture device 50 of FIGS. 2-4) positioned in series in the beam pathwith one rectangular LCD aperture device arranged to control waistposition in the resolving axis and the other rectangular LCD aperturedevice arranged to control waist position in the non-resolving axis. Byproviding them with independent control connection, such a dual aperturemechanism configuration permits independent control of waist position inboth the resolving and the non-resolving axes. Of course the twoaperture mechanisms could be controlled by a single signal and stillmodify waist position in both axes. Such a dual axis control systemwould be particularly useful in scanner applications which produces araster scan pattern, such as certain fixed scanners.

In another alternate embodiment, the amount of polarization of theincoming optical beam (i.e. the beam entering the aperture) may itselfbe adjusted. In this embodiment, the aperture mechanism may comprise anLCD (such as LCD aperture 250 in FIG. 7) or simply a sheet of polarizedmaterial having a central aperture of a configuration of the aperture250 of FIG. 7 where the regions 256a and 256b are merely polarizedmaterial. By varying the polarization of the incoming optical beam andpassing it through the polarized aperture, a grey-scale orincremental-type waist location adjustment may also be achieved. Onesuch embodiment is shown in FIG. 17 in which the aperture mechanism 850comprises a sheet of polarized material having a central aperture 852.The panels or bands 854, 854 (which be rectangular, round or some othershape) are polarized material. As the polarization of the incomingoptical beam 815 entering the aperture mechanism 850 is varied, theamount of light passing through the panels 854 is adjusted creating thegrey-scale waist adjustment. The polarization of the incoming light 15may be adjusted in several ways. The light source 810 itself may berotated about the axis of the beam 815 or may be electronicallycontrolled (for example by varying power frequency) to adjustpolarization. Alternately a polarization device 860 which adjusts thepolarization of light as it passes therethrough, may be disposedupstream of the aperture mechanism 850. The device 860 may be a liquidcrystal panel controllable by suitable means. Alternately the device 860may be a simple sheet of polarized material which is rotatable about theaxis of the beam 815 (in this instance a randomly polarized light source810 is preferred for efficiency reasons). Alternately, the aperturemechanism 850 (preferably having a circular central aperture to besymmetric) may be rotated about the axis of the beam 815.

The variable aperture mechanism described herein may also be used incombination with other variable-focus techniques. Such techniquesinclude for example (1) mechanical devices such as a focusing lensdesigned with an axially movable lens element to permit changing of theposition of the focal point, (2) the rotatable optical polygon mirrordevice of U.S. Pat. No. 4,560,862 which has a plurality of facets, eachmirror facet being of a different curvature, or (3) some activelychangeable focusing optical element.

The variable aperture mechanism (which controls beam waist position) mayalso be used to improve "speed-to-read" i.e. the time it takes thescanner to perform a successful scan. When the aperture controlledsystem obtains a successful scan, it has the information as to whataperture setting was used to accomplish the successful scan. Since aparticular user frequently will perform a number of similar tasksconsecutively, the desired focus distance is likely to be nearly thesame for consecutive scans. The control processor may then storeinformation as to a particular user (it is of note that a checkout clerktypically inputs an employee number to the register) or alternatelystore recent successful scan distances. The processor may then select apreferred starting aperture setting, for example the setting at the mostrecent successful scan. If in fact the user is scanning an item at asimilar distance to that of the previous item, time-to-read may beenhanced since the variable focus has an initial setting at a successfulscan distance.

Thus, an optical system and method capable of variable and/or selectablefocal planes have been shown and described. Though described withrespect to a preferred embodiment of an optical scanning device such asa bar code scanner, the device may be employed by other focusingmechanisms such as those employed by data transfer devices (such asreaders and encoders), particularly those using laser light such aslaser printer and compact disc technologies. Though certain examples andadvantages have been disclosed, further advantages and modifications maybecome obvious to one skilled in the art from the disclosures herein.The invention therefore is not to be limited except in the spirit of theclaims that follow.

What is claimed is:
 1. An optical system for data reading, comprising:alight source generating an optical beam along an outgoing optical pathtoward an object to be read; a focusing system positioned in theoutgoing optical path for focusing the optical beam to a given waist;means for adjusting position of the waist of the optical beam comprisingan aperture device positioned in the outgoing optical path, the aperturedevice forming an opening which is offset from a center of the outgoingbeam.
 2. An optical system according to claim 1 wherein the opening hasside edges which are irregular in shape to improve beam profile.
 3. Anoptical system according to claim 1 wherein the aperture formed by theaperture device is rectangular, its width being varied in a directioncorresponding to the scanning direction of the scanning apparatus.
 4. Anoptical system according to claim 1 wherein the opening formed by theaperture device is round, its width being variable in both the resolvingaxis and the non-resolving axis.
 5. An optical system according to claim1 wherein the opening formed by the aperture device is rectangular withits width being variable in a direction corresponding to the scanningdirection of the scanning apparatus.
 6. An optical system according toclaim 1 wherein the aperture device comprises a first aperture definedby a first opposing pair of liquid crystal regions.
 7. An optical systemaccording to claim 1 wherein the light source is selected from the groupconsisting of: lasers, laser diodes, coherent light sources, lightemitting diodes, non-coherent light sources, and combinations thereof.8. An optical system according to claim 1 wherein the optical systemcomprises a laser bar code scanner.
 9. An optical system according toclaim 1 wherein the aperture device comprises a plate with a centralaperture, the plate being rotatable about a center axis which isperpendicular to the outgoing optical path, wherein as the plate isrotated about its center axis to a desired degree, an effective width ofthe central aperture is projected.
 10. An optical system according toclaim 1 wherein the aperture device comprises a dual shutter aperturedevice including a first pivoting shutter element and a second pivotingshutter element defining an aperture therebetween, the aperture beingvariable corresponding to the degree of pivoting of the first and secondthe shutter elements.
 11. An optical system according to claim 1 whereinthe aperture device comprises a variable aperture mechanical iris. 12.An optical system according to claim 1 wherein the aperture devicecomprises a first sliding panel element and a second sliding panelelement defining an aperture therebetween, the first and second panelelements moving in tandem, to selectively vary aperture size.
 13. Anoptical system according to claim 1 wherein the aperture opening ispositioned on a side of the focusing system opposite to the lightsource.
 14. An optical system according to claim 1 wherein the apertureopening is positioned downstream of the focusing system.
 15. An opticalsystem according to claim 1 wherein the aperture opening is positionedupstream of the focusing system and adjacent thereto.
 16. An opticalsystem according to claim 1 wherein the aperture opening is positioneddownstream from the light source beyond a point one system focal lengthupstream of the focusing system.
 17. An optical system according toclaim 1 further comprisinga scanning apparatus for receiving the opticalbeam which has passed through the aperture device and for producing ascanning pattern of the optical beam toward the object; and a detectorfor detecting light reflecting and/or scattered off the object.
 18. Anoptical system according to claim 17 wherein the aperture opening ispositioned between the focusing system and the scanning mechanism. 19.An optical system according to claim 1 wherein the opening of theaperture device is variable in width.
 20. An optical system according toclaim 19 wherein the opening is of a size smaller than a diffractivelimit of the optical beam.
 21. An optical system according to claim 1wherein the aperture device comprises liquid crystal regions defining atleast one gate opening of a width along a scanning direction, whereinactivation of certain liquid crystal regions rotates the polarization oflight passing through the certain liquid crystal regions.
 22. An opticalsystem according to claim 21 further comprising a polarizer positioneddownstream of the aperture device which in combination with the liquidcrystal regions selectively vary width of the gate opening.
 23. Anoptical system according to claim 22 wherein the polarizer comprises apolarizing beam splitter acting also as a fold mirror to direct theoptical beam passing through the aperture device to a scanningmechanism.
 24. An optical system according to claim 22 wherein theaperture device further comprises a second gate opening of a secondwidth defined by at least one inner liquid crystal panel locatedadjacent and radially inward of one said first pair of liquid crystalpanels wherein the first pair of liquid crystal panels and the innerliquid crystal panel are selectively activatable and wherein activationthereof selects width of the opening.
 25. An optical system according toclaim 21 wherein the liquid crystal regions comprise a pair of liquidcrystal panels positioned about a central axis offset from the center ofthe optical beam, thereby defining an opening of a first width.
 26. Anoptical system according to claim 21 further comprising a controllerconnected to the liquid crystal regions for activating the liquidcrystal regions to a desired activation level thereby selecting a degreeto which the optical beam is polarized when passing through the liquidcrystal regions thereby enabling the waist to be positioned at a desiredlocation.
 27. An optical system according to claim 26 wherein thecontroller controls polarization by adjusting electrical voltage appliedto the liquid crystal regions.
 28. An optical system according to claim26 wherein the controller controls polarization by adjusting electricalfrequency applied to the liquid crystal regions.
 29. A laser scanner forbar code reading, comprising:a laser diode generating a laser beam alongan outgoing optical path; focusing lens positioned in the outgoingoptical path for focusing the laser beam to a waist at a given location;an aperture device forming an aperture opening of a given width foradjusting location of the waist, the aperture opening positioned in theoutgoing optical path at a position downstream from one focal lengthupstream of the focusing lens, wherein the aperture opening isnon-symmetrically positioned to one side of a center of the outgoingoptical beam.
 30. In a data reader having a laser diode module producinga laser beam along an outgoing optical path and an aperture mechanismwith an aperture opening disposed in the optical path, a method forassembling the data reader comprising the steps of:determining to whichside from a center of the outgoing optical path that locating theaperture opening offset from the center produces superior beam profile;and arranging the aperture mechanism and the laser diode module suchthat the aperture opening is non-symmetrically positioned to that sideof the center of the outgoing optical path.
 31. A method according toclaim 30 wherein the aperture opening comprises a gate which aperturesthe laser beam along one axis, wherein the step of arranging comprisesrotating the laser diode module 180° if necessary to orient the laserbeam relative to the aperture opening.
 32. A method according to claim30 wherein the aperture opening comprises a gate which apertures thelaser beam along one axis, wherein the step of arranging comprisesrotating the aperture mechanism 180° if necessary to align the apertureopening relative to the laser beam.
 33. In a data reader having a laserdiode module producing a laser beam along an outgoing optical path andan aperture mechanism with an aperture opening disposed in the opticalpath, a method for assembling the data reader comprising the stepsof:(a) testing the laser diode module to determine to which side of thelaser beam locating the aperture opening offset to produces a superiorbeam profile; (b) aligning the laser diode module and the aperturemechanism on the basis of the result of the testing in step (a).
 34. Amethod according to claim 33 further comprisingtagging the laser diodemodule with a marking as to that side determined in step (a) andorienting and installing the laser diode module and the aperture openingrelative to each other according to the marking.
 35. A method accordingto claim 33 further comprising(c) installing a focusing lens in theoptical path to focus the laser beam to a given waist location; (d)locating the aperture opening downstream of a point one focal lengthupstream of the focusing lens.
 36. An optical system for data reading,comprising:a light source generating an optical beam along an outgoingoptical path; a focusing system positioned in the outgoing optical pathfor focusing the optical beam to a waist at a given location; anaperture device positioned in the outgoing optical path, the aperturedevice forming an aperture opening having side edges which are jagged inshape.
 37. An optical system according to claim 36 wherein the aperturedevice is positioned downstream from the focusing system.
 38. An opticalsystem according to claim 36 wherein the aperture device is positioneddownstream from the light source beyond a point one system focal lengthupstream of the focusing system.
 39. An optical system according toclaim 36 wherein the aperture device comprises a gate of a given widthfor aperturing the optical beam along one axis.